# Homework Help: Reynold number

1. Feb 15, 2017

### fonseh

1. The problem statement, all variables and given/known data
https://en.wikipedia.org/wiki/Reynolds_number#Flow_in_a_pipe

In this link , i was told that Reynold number = ρvD / μ ,

However in the notes below , i was told that Reynold number = ρvR / μ .

2. Relevant equations

3. The attempt at a solution
I think Reynold number = ρvR / μ is wrong , because i learnt that Reynold number = ρvD / μ in fluid mechanics , however , in the hydraulics notes , i was told that that Reynold number = ρvR / μ . i'm very confused now

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2. Feb 15, 2017

### BvU

I agree. The characteristic length is the hydraulic diameter and it is confusing to use the term "hydraulic radius".

3. Feb 15, 2017

### Staff: Mentor

It doesn't really matter, as long as the meaning is established within the context of the system being analyzed. Anyway, it's Reynolds number, not Reynold number.

4. Feb 15, 2017

### fonseh

So , which is correct ? ρvD / μ or ρvR / μ ??

5. Feb 15, 2017

### fonseh

So , do you mean ρvD / μ is correct ? and ρvR / μ is wrong ?

6. Feb 15, 2017

### Staff: Mentor

Both are correct, depending on how it is used.

7. Feb 15, 2017

### fonseh

Can you explain further when we need to use ρvD / μ or ρvR / μ ???

8. Feb 15, 2017

### Staff: Mentor

In a given problem, you just need to specify which definition is being used. For example, the laminar-turbulent transition occurs at $\rho v D/\mu=2100$ for flow in a pipe, but it also occurs at $\rho v R/\mu=1050$. So you have to tell which definition of the Re you are using.